Hollow valves for internal combustion engines have been part of the prior art for a long time and, owing to the cooling effect, make possible a longer service life and improved performance of the internal combustion engine. In addition to simple heat dissipation via a valve seat ring in the cylinder head, such hollow valves also offer the possibility of accommodating a cooling medium, for example sodium, in the cavity thereof, as a result of which advantages can be achieved in particular with use as an outlet valve. Sodium melts at 97.5 degrees Celsius and is also a very good conductor of heat, the sodium becoming liquid owing to the operating temperature of the internal combustion engine and then being moved to and fro in the stem by the mass forces. This is also referred to as a “shaker effect”. The coolant, that is, usually the sodium, transports some of the heat produced during combustion from the valve disc and in particular from a highly loaded hollow throat region into the stem region and can be conducted away from there via the valve guide. The temperatures at the valve disc can be reduced by 80° C. to 150° C. by means of sodium-cooled hollow valves. A positive side effect is also that such a hollow valve is more lightweight than a valve with a solid valve stem, despite the sodium filling.
In addition to the actual bore in the valve stem, however, a cavity in the valve disc should be greater in order to be able to cool the highly loaded regions of the valve disc (for example hollow throat region) better. What is known as the ECM method (electrochemical machining) is usually used to do this, in which a cathode is introduced into the bore of the valve stem and emits electrons via an electrolyte to the workpiece, in this case the hollow valve, which acts as an anode.
The disadvantage of the known hollow valves is however their comparatively complex production, since extremely precise guiding and positioning of the cathode is necessary to produce the cavity provided in the valve disc or valve head.